A basic commercial process for recovery of hydrogen sulfide from acid gas streams is the Claus process. The basic chemical reactions occurring in the Claus process are presented in the following equations. EQU H.sub.2 S+1/2O.sub.2 .revreaction.H.sub.2 O+S (I) EQU H.sub.2 S+3/2O.sub.2 .revreaction.H.sub.2 O+SO.sub.2 (II) EQU H.sub.2 S.revreaction.H.sub.2 +S (III) EQU 2H.sub.2 S+SO.sub.2 .revreaction.2H.sub.2 O+3S (IV)
Reactions (I), (II), and (III) occur in a thermal zone (reaction furnace) sometimes called a muffle furnace, and reaction (IV) occurs primarily in a catalytic zone (catalytic reactor). The Claus process thus comprises two stages: (1) a thermal stage operated above, for example, 1850.degree. F., and typically in the range of 2000.degree. F. to 3000.degree. F. and (2) a catalytic stage generally, for example, operated between a temperature somewhat above the sulfur dewpoint of the gas stream and about 700.degree. F.
In the thermal zone, a gas stream containing hydrogen sulfide can be burned with an oxidant in a reaction furnace to form hot combustion gases containing a substantial amount of free sulfur which can be condensed after cooling the hot combustion gases. Part of the heat of combustion is recovered in a waste heat boiler and sulfur condensed in a sulfur condenser. Thereafter, the lean gas goes to the catalytic reactor system wherein the reaction of Equation (IV) takes place.
Typically, the oxidant is air. However, other oxidants which are suitable include pure oxygen, oxygen enriched air, and sulfur dioxide.
As is well known, the flame temperature in the furnace is a function of the composition of the acid gas. As the hydrogen sulfide content of this gas decreases, the flame temperature also decreases. A flame temperature less than 1850.degree. F. is usually somewhat unstable and a minimum temperature of about 2000.degree. F. is normally recommended. A number of proposals have been made to improve the treatment of low-quality gas streams. One system includes preheating either or both of the products to be combusted, the hydrogen sulfide stream and the oxidant stream. A difficulty in this system does occur since preheating the acid gas above about 600.degree. F. creates a problem as metals above this temperature tend to corrode rapidly when in contact with the gas. The air fed to the furnace can be preheated as high as 900.degree. F. The following table showing flame temperatures is based on an acid gas containing the amount set forth of hydrogen sulfide and carbon dioxide and three percent (3%) water, one and a half percent (1.5%) methane, and one-half percent (0.5%) ethane, all amounts being set forth on a volume basis, in a straightthrough Claus plant and an ambient air temperature of 90.degree. F.
______________________________________ Acid Gas Air Temperature Flame % H.sub.2 S % CO.sub.2 Temp. Into Furnace Temp. ______________________________________ 60.0 35.0 80.degree. F. 200.degree. F. 2016.degree. F. 50.0 45.0 80.degree. F. 200.degree. F. 1880.degree. F. 60.0 35.0 300.degree. F. 300.degree. F. 2156.degree. F. 50.0 45.0 300.degree. F. 300.degree. F. 2021.degree. F. 40.0 55.0 300.degree. F. 300.degree. F. 1888.degree. F. 40.0 55.0 600.degree. F. 900.degree. F. 2241.degree. F. 30.0 65.0 600.degree. F. 900.degree. F. 2053.degree. F. 20.0 75.0 600.degree. F. 900.degree. F. 1803.degree. F. ______________________________________
Thus, with decreasing H.sub.2 S content, it becomes difficult or impossible to maintain a stable flame. Where concentrations are so low that preheating is impractical, it has been suggested that supplemental fuel be either added to the acid gas to increase its heating value or to be burned in an auxiliary burner in the furnace to provide the additional heat which is required. The fuel gas combustion products from either of these methods is mixed with the process gas, thereby diluting the process gas and reducing the recovery. Further, this dilution requires larger process equipment in the downstream treating sections.
An object of this invention is to provide a process for treating low-quality acid gas streams.
A further object of this invention is to provide a furnace for carrying out this process.
Other objects and advantages of the invention will be apparent to one skilled in the art upon reading the accompanying disclosure.